After much research I purchased the "Maine Roll On" from Maine Outdoor Products, which is a big help in loading and unloading my canoe. I attach my "Maine Roll On" to my Barrecrafters bar rack (I have also posted a review of this bar rack and companion canoe carrier). The Maine Roll On is also designed to fit Thule and Yakima systems.
The roller is well made and the canoe glides up and onto and off of the rack with ease. The unit has two loading/unloading positions and a transport position.
I highly recommend this product for those who need to load and unload their canoe or kayak without the assistance of others.